Nonlinear-optical interactions in fluorescein-doped boric acid glass

Fluorescein-doped boric acid glass is a material characterized by an extremely low saturation intensity of ∼15 mW?^-2 and a nonlinear susceptibility χ⁽³⁾ as large as ∼1 esu. The saturated absorption of this material is shown both theoretically and experimentally to depend on the state of polarization of the saturating beam, even though the unsaturated absorption is polarization insensitive. Phase-conjugate reflectivities as large as 0.6% have been obtained through use of degenerate four-wave mixing in this material. These measured reflectivities are in good agreement with the predictions of a theory that includes the effects of excited-state absorption and grating washout. In addition, two-beam coupling due to the nonlinearity of saturable absorption has been demonstrated in this material. The magnitude of the coupling is maximized by inducing a frequency shift between the two beams of ∼0.1 Hz.